Red Sun dying.

The article discusses how the discovery that planets around other stars are common will forever stand as one of the great achievements of astronomy in the 1990s. Given the run-of-the-mill nature of the Sun, I'd expected that the processes that led to the formation of planets in our solar system would naturally operate in other locales. What did surprise [and please] me, however, was learning that other solar systems would exhibit many exotic architectures. In our own solar system, we know the Sun is about halfway through its 12-billion-year "main sequence" phase. This phase will end when the Sun exhausts its supply of usable hydrogen fuel. To calculate what will happen to the outer solar system when the Sun goes red giant, we need to know the Sun's predicted mass, size, and luminosity over time. We also need a mathematical recipe for calculating planetary temperatures using several variables, including the planet's distance from the Sun, the Sun's luminosity, and the planet's properties. Information on the Sun's luminosity evolution is published in scientific literature. It's based on stellar evolution calculations that involve such criteria as nuclear fusion rates that control the rate of energy generation in the Sun's core, and the response of the Sun's outer layers to those internal processes. We can use luminosity projections of the red giant Sun to calculate the approximate surface temperature of planets around the Sun [or another star] knowing only the distance of the planet from its star and the reflectivity of the planet. For a common star like the Sun, the room temperature distance moves outward late in life from the 1 astronomical unit [AU] distance we known and love here on Earth to as far as 50 AU. Similar, but more extreme, behavior is exhibited in stars with higher masses. But the basic message remains unchanged: The habitable zone around each star will move outward as the star gets older, moving from the inner to the outer solar system.